High dielectric constant materials

ABSTRACT

A method and structure for an integrated circuit structure that includes introducing precursors on a substrate, oxidizing the precursors and heating the precursors. The introducing and the oxidizing of the precursors is preformed in a manner so as to form an amorphous glass dielectric on the substrate. The process preferably includes, before introducing the precursors on the substrate, cleaning the substrate. The introducing of precursors is performed in molar ratios consistent with formation of glass films and may comprise an atomic level chemical vapor deposition of La 2 O 3  and Al 2 O 3  using ratios between  20 %- 50 % La 2 O 3  and  50 %- 80 % Al 2 O 3 .

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention generally relates to the formation ofdielectric materials in semiconductor integrated circuits and moreparticularly to an improved dielectric material and process for formingthe same.

[0003] 2. Description of the Related Art

[0004] Current integrated circuit processing technology, such as trenchcapacitor technology, utilizes thin films of oxidized silicon nitride asthe node dielectric responsible for charge storage in the dynamic randomaccess memory (DRAM) cell. As ground rules of the devices continue toshrink however, maintaining the minimum cell capacitance becomesincreasingly difficult and to this point, has been achieved via athinning of the node nitride. An alternative approach is to increase thedielectric constant (k) of the node material via the utilization of analternative oxide with higher K (e.g. Ta₂O₅, Al₂O₃, ZrO₂, BSTO, etc.).Although higher dielectric constants can be achieved, processingtemperatures in excess of 1000° C. are commonly required for such highk-trench capacitor DRAM'S. The elevated processing temperatures andextended soak times required at such temperatures commonly result incrystallization of the dielectric and excessive leakage along theresultant grain boundaries.

[0005] Therefore, there is a need for an improved dielectric that can beused with conventional semiconductor processing and that does notcrystalize at processing temperatures in excess of 1000° C. Theinvention discussed below presents such a dielectric.

SUMMARY OF THE INVENTION

[0006] In view of the foregoing and other problems, disadvantages, anddrawbacks of the conventional dielectric materials, the presentinvention has been devised, and it is an object of the present inventionto provide a structure and method for an improved dielectric material.In order to attain the object(s) suggested above, there is provided,according to one aspect of the invention a process of forming adielectric in an integrated circuit structure that includes introducingprecursors on a substrate, oxidizing the precursors and heating theprecursors. The introducing and the oxidizing of the precursors ispreformed in a manner so as to form an amorphous glass dielectric on thesubstrate. The process preferably includes, before introducing theprecursors on the substrate, cleaning the substrate. The introducing ofprecursors is performed in molar ratios consistent with formation ofglass films and may be achieved using ALCVD or MOCVD (i.e. Atomic layerCVD or Metal-organic CVD) of La₂O₃ and Al₂O₃ using ratios between20%-50% La₂O₃ and 50%-80% Al₂O₃.

[0007] The invention produces a dielectric that remains amorphous up toat least 1000° C. and that has a dielectric constant of approximately20. Therefore, the dielectric produced with the invention remainsamorphous at elevated temperatures and thus maintains low electricalleakage via the elimination of grain boundaries.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The foregoing and other objects, aspects and advantages will bebetter understood from the following detailed description of a preferredembodiment(s) of the invention with reference to the drawings, in which:

[0009]FIG. 1 is a flow diagram illustrating a preferred method of theinvention; and

[0010]FIG. 2 is a schematic diagram of the integrated circuit structureproduced with the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

[0011] As discussed above, there is a need for an improved dielectricthat can be used with conventional semiconductor processing and thatdoes not crystalize at processing temperatures up to 1000° C. In orderto maintain the elevated dielectric constant of the material and keepthe electrical leakage to a minimum, the invention uses a process toform glass films (i.e. amorphous films) which maintain theirnon-crystalline structure at the elevated temperatures required in thefabrication of integrated circuit devices, such as deep trenchcapacitors.

[0012] This invention raises the dielectric constant of the nodedielectric via the utilization of mixtures of high-K oxides in molarratios consistent with the formation of glass films. Such films areintended to remain amorphous at elevated temperatures and thus maintainlow electrical leakage via the elimination of grain boundaries. Forexample, in the bulk, La₂O₃ and Al₂O₃ can be mixed in a ratio of40La₂O₃+60Al₂O₃ to form a transparent glass with a critical cooling rateof less than 100 K/s. Thus, when deposited from the vapor phase, thecooling rate achieved with the inventive dielectric should be well inexcess of the 100 K/s required to keep the glass vitreous (e.g.,non-crystalline).

[0013] Furthermore, because the dielectric is amorphous a linear mixinglaw applies, which allows the dielectric constant of the resulting glassto be as great as 20 (the dielectric constants of La₂O₃ and Al₂O₃ areapproximately 30 and 10, respectively) and thus more than a factor of 4better than oxidized silicon nitride (which has a dielectric constant ofapproximately 4), and still larger than that of pure Si₃N₄ (k˜7).Although, the fabrication of a two component glass is preferable from anease of processing standpoint, it should be noted that the addition of athird or fourth oxide (e.g. SiO₂) is possible with the invention andcould in fact be beneficial.

[0014] Mixing of oxides have been suggested for the fabrication ofconventional gate oxides. In such cases, mixed oxides are formed asbinary alloys consisting of dilute quantities of some high-k material(e.g. ZrO₂) with the balance made up of SiO₂ (generically referred to as“silicates”). In contrast, the glass films of the invention do not haveto adhere to the stringent interfacial requirements of a CMOS device andas such they can employ oxides other than SiO₂. Such compositions lendthemselves to enhanced dielectric characteristics applicable to DRAMstorage devices.

[0015] As mentioned above, the implementation of a high-k material isdesirable in integrated circuit technology, such as trench capacitortechnology. Although there are numerous conventional materials fromwhich to choose, most (e.g. ZrO₂, HfO₂) crystallize at temperaturesbelow 1000° C. Because capacitor construction is a front end process,most high-k films will therefore devitrify (i.e., loose their amorphousstructure and crystalize) and be susceptible to excessive electricalleakage and premature failure. It is important therefore to design amaterial with an appropriate dielectric constant which remains amorphousat temperatures up to (and in excess of) 1000° C. To that end, an atomiclayer chemical vapor deposition (ALCVD) process is described here inwhich glass forming combinations of La₂O₃ and Al₂O₃ are deposited foruse as a high-k dielectric in trench capacitor technologies.

[0016] More specifically, referring to the flowchart in FIG. 1 andschematic deposition chamber 22 in FIG. 2, a ALCVD deposition ofLa₂O₃—Al₂O₃ glasses is shown. In item 10, the surface of the material 20on which the dielectric is to be formed (e.g., silicon substrate,integrated circuit, etc.) is prepared and cleaned. In such surfacepreparation, the invention terminates the substrate with Si—O bonds.SC1/SC2 or Huang A-B type cleans are examples of well-known processesthat can be used here.

[0017] Precleans are typically used to remove residual organic materialsfrom the surface or strip the native oxide that develops on wafersurfaces upon exposure to atmosphere. The surface treatment in the casedescribed here prepares the surface with a fresh chemical oxide (Si—Obonds) thereby improving the deposition characteristics of the processand thus the electrical properties of the resultant film. There areextensive variations on surface pretreatments that might be employed andthe SC1/Huang type mentioned above are meant to serve only asillustrations of applicable processes.

[0018] In item 11, the deposition chamber 22 is set to temperature lessthan 500° C. Typical ALCVD type processes utilizes wafer temperaturesfrom 100-500° C. The temperature for each process is dependent upon theprecursor materials used. It should be noted that there are variationson thermally driven ALD (i.e. the driving force for film deposition isthe chemical reaction between precursor A and precursor B, which isstrongly dependent on temperature) that serve to reduce the wafertemperature required, e.g. plasma enhanced ALD.

[0019] In item 12, the invention introduces precursors 24 (e.g., La, Al,Zr, Ta, Hf, Ti, etc.) into the deposition chamber. The precursors areintroduced sequentially (i.e., an A-B-A-B type process) in backgroundpressures of inert gas (N2 or Ar) on order of 1 Torr. Base pressures inthe chamber prior to starting film deposition are ˜1×10 ⁻⁶ Torr.

[0020] When introducing the precursors, it is important that the ratioof cations be consistent with glass formation. Historically, there aremixtures of oxides which resist crystallization to high temperatures.Deviations from these glass forming compositions results in theformation of crystalline phases. Based on the phase diagrams for theoxides of interest in this invention therefore (La₂O₃, ZrO₂, HfO₂, TiO₂,Ta₂O₅, SiO₂, Al₂O₃, etc.) there are observed a number of glass formingregions. As an example, recent work has been conducted on the La₂O₃,Al₂O₃ and SiO₂ glasses. As reported by Caurant et al. (Eur. J. SolidState Inorg. Chem., V. 30, 1993, 307-34 and V. 29, 1992, 1205-1215) theglassy phase in the Al₂O₃—La₂O₃ system extends roughly from 80% to 50%Al₂O₃ (the balance La₂O₃). Additional work conducted more recently byWeber et al. (J. Am. Ceram. Soc., 83, 1868-1872, 2000) is consistentwith that range and reports glass formation at 60 mol % Al₂O₃ and 40 mol%La₂O₃. The addition of more components to the mix is sometimes desiredto increase the temperature stability, adjust chemical behavior, orengineer the optical properties of glasses. In such a case (e.g., viathe addition of SiO₂) the molar ratios will change (e.g., N. Clayden etal., J. Non-Crystalline Solids, V. 258, 1999, 11-19) and such changeswould be reflected in synthesis of the DRAM dielectric.

[0021] The chamber is preferably purged with an inert gas (such as N2)between pulses of the precursor material. In item 13, the inventiveprocess oxidizes the precursors (a commonly used oxidant is water vapor,ozone, nitric oxide, nitrous oxide etc.) and the invention allows theoxidation reaction to proceed to completion.

[0022] As shown in item 14, the inventive process then checks to see ifthe oxidized film 24 has achieved the necessary thickness. If not, theprocessing in items 12 and 13 is repeated until the film has achievedthe necessary thickness. Typical thicknesses will likely range from20-200A.

[0023] Next, in item 15, a rapid thermal processing (RTP) is performedin the chamber 22 in an inert (N2, Ar) or oxidizing (O₂, N₂O, NO, H₂O)ambient to homogenize the composition. The invention then cools thechamber 22 in a process that avoids crystallization of the glass 24. Inthe inventive process, the homogenization temperature is expected to bebetween 500° C-1000° C. Critical cooling rates (the minimum rate atwhich a glass must be cooled to remain amorphous) are dependent upon amaterial's composition and typically fall between 25° C-50° C./min.

[0024] Finally, the remaining conventional processes necessary tocomplete the remaining integrated circuit chip device are preformed initem 16. Further, the processing described above can be supplementedwith any additional processing required by specific structures. Forexample, if items 20, 24 are a deep trench storage array, the substrate20 may be patterned with deep trenches before the formation of thedielectric 24 and a conductor would fill the trenches above thedielectric 24. Therefore, while the schematic diagram in FIG. 2 merelyrepresents a cross-sectional view of two surfaces 20, 24, as would beknown by one ordinarily skilled in the art, such a drawing actuallyrepresents any type of integrated circuit device ranging from a smallcapacitor, to an entire integrated circuit chip. Such processing iswell-known to those ordinarily skilled in the art and will not bediscussed in detail herein, or shown in the drawings, so as to notunnecessarily obscure the salient features of the invention.

[0025] While an ALCVD process is discussed above for the synthesis ofLa₂O₃—Al₂O₃ based glass films, the inventive process is not limited toatomic layer techniques. Both metal-organic CVD (MOCVD) and chemicalsolution deposition (CSD) are alternative methods capable of depositingfilms with the desired composition that work well with the invention.For example, films with approximate compositions of 84 mol % aluminumoxide and 16% lanthanum oxide can be deposited on single crystal siliconsubstrates. X-ray diffraction can be used to characterize the films as afunction of annealing temperature. An important feature of the inventionis that the dielectric material remains amorphous at temperatures up to(and sometimes slightly in excess of) 1100° C.

[0026] The invention described above can also be modified to accommodatespecific structures. For example, with respect to deep trench storagestructures, although an extensive number of high-K dielectric materialcandidates exists (e.g. TiO₂, BSTO, Ta₂O₅, etc.), there are two primarycriteria which must be satisfied prior to integration in deep trenchcapacitors. The first is that the material must conform to the shape ofthe trench and the second is thermal stability. Both La₂O₃ and Al₂O₃ canbe conformally deposited by ALCVD. By engineering the exposure times(i.e. the pulse lengths) or delivery mechanism 12 of the precursors 24,different cation ratios can be achieved and the required glass formingcompositions deposited.

[0027] Aside from the conformality requirements, and because of theelevated temperatures of subsequent front end processes (>1000° C.),alternative storage dielectrics must not react with, or diffuse into,the silicon trench on which they are deposited. Both Al₂O₃ and La₂O₃ arechemically stable in contact with silicon to 1000° C. Furthermore,mixtures of the two oxides remain vitreous to temperatures well inexcess of 1100° C.

[0028] With the invention, the dielectric constant of the material 24 isincreased when compared to conventional NO insulators. For example, NOhas a k of 4; ZrO₂ and HfO₂ have a k of 20, BSTO has a k of 200. The kof the inventive La₂O₃—Al₂O₃ is greater than 10 and is approximately 20.Further, the inventive films remain amorphous at temperatures up to1000° C. (and above). Other high dielectric materials crystallize attemperatures less than 1000° C., which leads to unwanted electricalleakage and reliability problems.

[0029] The invention also promotes enhanced scaleability because theinventive film composition can be changed with each generation todecrease EOT (equivalent oxide thickness) and maintain physicalthickness. Therefore, with the invention, the learning gained fromprevious generations is not completely lost. The film composition (i.e.the ratio of cations to anions in the films) as well as the thicknesscan be continually adjusted to increase the dielectric constant, in turndecreasing the EOT and allowing continued use of the material forseveral generations. This avoids having to completely replace thedielectric material with a new high-k material as the scalinggenerations progress. High-k materials with set ratios can only bescaled in thickness, not composition.

[0030] Upon development of processes to synthesize the glass films usedin this application, those processes and materials sets might find usein other aspects of microelectrics. For example, the wet and dry etchcharacteristics might be changed via an adjustment of the molar ratiosof the constituent oxides. Changes in glass composition might also findutility in lithographic applications where changes in the refractiveindex and optical absorption are important. In such a case changing thefilm's composition might lead to desired lithographic performance.Additional use might be realized in back-end passive applications wheredielectric materials are used in the construction of both capacitors andresistors.

[0031] While the invention has been described in terms of preferredembodiments, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theappended claims.

What is claimed is:
 1. A process of forming a high-k dielectric in anintegrated circuit structure comprising: introducing precursors on asubstrate; oxidizing said precursors; and heating said precursors,wherein said introducing and said oxidizing of said precursors ispreformed in a manner so as to form an amorphous glass dielectric onsaid substrate.
 2. The process in claim 1, wherein said introducing ofsaid precursors is performed in molar ratios consistent with formationof glass films.
 3. The process in claim 1, wherein said introducing ofsaid precursors comprises an atomic level chemical vapor deposition ofLa₂O₃ and Al₂O₃.
 4. The process in claim 1, wherein said an atomic levelchemical vapor deposition comprises between 20%-50% La₂O₃ and 50%-80%Al₂O₃.
 5. The process in claim 1, further comprising, before saidintroducing of said precursors on said substrate, cleaning saidsubstrate.
 6. The process in claim 1, wherein said introducing and saidoxidizing of said precursors produces a dielectric that remainsamorphous up to 1000° C.
 7. The process in claim 1, wherein saidintroducing and said oxidizing of said precursors produces a dielectricthat has a dielectric constant of approximately
 20. 8. A process offorming an insulator for a deep trench capacitor in an integratedcircuit structure comprising: forming deep trenches in a substrateintroducing precursors on said substrate; oxidizing said precursors; andheating said precursors, wherein said introducing and said oxidizing ofsaid precursors is preformed in a manner so as to form an amorphousglass dielectric on said substrate.
 9. The process in claim 8, whereinsaid introducing of said precursors is performed in molar ratiosconsistent with formation of glass films.
 10. The process in claim 8,wherein said introducing of said precursors comprises an atomic levelchemical vapor deposition of La₂O₃ and Al₂O₃.
 11. The process in claim8, wherein said an atomic level chemical vapor deposition comprisesbetween 20%-50% La₂O₃ and 50%-80% Al₂O₃.
 12. The process in claim 8,further comprising, before said introducing of said precursors on saidsubstrate, cleaning said substrate.
 13. The process in claim 8, whereinsaid introducing and said oxidizing of said precursors produces adielectric that remains amorphous up to 1000° C.
 14. The process inclaim 8, wherein said introducing and said oxidizing of said precursorsproduces a dielectric that has a dielectric constant of approximately20.
 15. A dielectric comprising: an oxide material having molar ratiosconsistent with amorphous glass.
 16. The dielectric in claim 15, whereinsaid oxide material comprises between 20%-50% La₂O₃ and 50%-80% Al₂O₃.17. The dielectric in claim 15, wherein said oxide material has acritical cooling rate of less than 100° k/s.
 18. The dielectric in claim15, wherein said oxide material has a dielectric constant as high as 20.19. The dielectric in claim 15, wherein said oxide material remainsamorphous up to 1100° C.
 20. The dialectic in claim 15, wherein adielectric constant of said oxide material comprises a combination ofsaid molar ratios of material components of said oxide material.